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Diffstat (limited to 'Documentation/howto/revert-a-faulty-merge.txt')
| -rw-r--r-- | Documentation/howto/revert-a-faulty-merge.txt | 128 |
1 files changed, 111 insertions, 17 deletions
diff --git a/Documentation/howto/revert-a-faulty-merge.txt b/Documentation/howto/revert-a-faulty-merge.txt index 39b1da440a..19f59cc888 100644 --- a/Documentation/howto/revert-a-faulty-merge.txt +++ b/Documentation/howto/revert-a-faulty-merge.txt @@ -7,6 +7,10 @@ Abstract: Sometimes a branch that was already merged to the mainline after the offending branch is fixed. Message-ID: <7vocz8a6zk.fsf@gitster.siamese.dyndns.org> References: <alpine.LFD.2.00.0812181949450.14014@localhost.localdomain> +Content-type: text/asciidoc + +How to revert a faulty merge +============================ Alan <alan@clueserver.org> said: @@ -26,31 +30,31 @@ The history immediately after the "revert of the merge" would look like this: ---o---o---o---M---x---x---W - / + / ---A---B where A and B are on the side development that was not so good, M is the merge that brings these premature changes into the mainline, x are changes unrelated to what the side branch did and already made on the mainline, and W is the "revert of the merge M" (doesn't W look M upside down?). -IOW, "diff W^..W" is similar to "diff -R M^..M". +IOW, `"diff W^..W"` is similar to `"diff -R M^..M"`. Such a "revert" of a merge can be made with: $ git revert -m 1 M -After the develpers of the side branch fixes their mistakes, the history +After the developers of the side branch fix their mistakes, the history may look like this: ---o---o---o---M---x---x---W---x - / + / ---A---B-------------------C---D where C and D are to fix what was broken in A and B, and you may already have some other changes on the mainline after W. If you merge the updated side branch (with D at its tip), none of the -changes made in A nor B will be in the result, because they were reverted +changes made in A or B will be in the result, because they were reverted by W. That is what Alan saw. Linus explains the situation: @@ -77,7 +81,7 @@ In such a situation, you would want to first revert the previous revert, which would make the history look like this: ---o---o---o---M---x---x---W---x---Y - / + / ---A---B-------------------C---D where Y is the revert of W. Such a "revert of the revert" can be done @@ -86,17 +90,17 @@ with: $ git revert W This history would (ignoring possible conflicts between what W and W..Y -changed) be equivalent to not having W nor Y at all in the history: +changed) be equivalent to not having W or Y at all in the history: ---o---o---o---M---x---x-------x---- - / + / ---A---B-------------------C---D and merging the side branch again will not have conflict arising from an earlier revert and revert of the revert. ---o---o---o---M---x---x-------x-------* - / / + / / ---A---B-------------------C---D Of course the changes made in C and D still can conflict with what was @@ -107,25 +111,25 @@ faulty A and B, and redone the changes on top of the updated mainline after the revert, the history would have looked like this: ---o---o---o---M---x---x---W---x---x - / \ + / \ ---A---B A'--B'--C' If you reverted the revert in such a case as in the previous example: ---o---o---o---M---x---x---W---x---x---Y---* - / \ / + / \ / ---A---B A'--B'--C' -where Y is the revert of W, A' and B'are rerolled A and B, and there may -also be a further fix-up C' on the side branch. "diff Y^..Y" is similar -to "diff -R W^..W" (which in turn means it is similar to "diff M^..M"), -and "diff A'^..C'" by definition would be similar but different from that, +where Y is the revert of W, A' and B' are rerolled A and B, and there may +also be a further fix-up C' on the side branch. `"diff Y^..Y"` is similar +to `"diff -R W^..W"` (which in turn means it is similar to `"diff M^..M"`), +and `"diff A'^..C'"` by definition would be similar but different from that, because it is a rerolled series of the earlier change. There will be a lot of overlapping changes that result in conflicts. So do not do "revert of revert" blindly without thinking.. ---o---o---o---M---x---x---W---x---x - / \ + / \ ---A---B A'--B'--C' In the history with rebased side branch, W (and M) are behind the merge @@ -142,6 +146,8 @@ different resolution strategies: revert of a merge was rebuilt from scratch (i.e. rebasing and fixing, as you seem to have interpreted), then re-merging the result without doing anything else fancy would be the right thing to do. + (See the ADDENDUM below for how to rebuild a branch from scratch + without changing its original branching-off point.) However, there are things to keep in mind when reverting a merge (and reverting such a revert). @@ -158,7 +164,7 @@ merged. So it's debugging hell, because now you don't have lots of small changes that you can try to pinpoint which _part_ of it changes. But does it all work? Sure it does. You can revert a merge, and from a -purely technical angle, git did it very naturally and had no real +purely technical angle, Git did it very naturally and had no real troubles. It just considered it a change from "state before merge" to "state after merge", and that was it. Nothing complicated, nothing odd, nothing really dangerous. Git will do it without even thinking about it. @@ -177,3 +183,91 @@ the answer is: "oops, I really shouldn't have merged it, because it wasn't ready yet, and I really need to undo _all_ of the merge"). So then you really should revert the merge, but when you want to re-do the merge, you now need to do it by reverting the revert. + +ADDENDUM + +Sometimes you have to rewrite one of a topic branch's commits *and* you can't +change the topic's branching-off point. Consider the following situation: + + P---o---o---M---x---x---W---x + \ / + A---B---C + +where commit W reverted commit M because it turned out that commit B was wrong +and needs to be rewritten, but you need the rewritten topic to still branch +from commit P (perhaps P is a branching-off point for yet another branch, and +you want be able to merge the topic into both branches). + +The natural thing to do in this case is to checkout the A-B-C branch and use +"rebase -i P" to change commit B. However this does not rewrite commit A, +because "rebase -i" by default fast-forwards over any initial commits selected +with the "pick" command. So you end up with this: + + P---o---o---M---x---x---W---x + \ / + A---B---C <-- old branch + \ + B'---C' <-- naively rewritten branch + +To merge A-B'-C' into the mainline branch you would still have to first revert +commit W in order to pick up the changes in A, but then it's likely that the +changes in B' will conflict with the original B changes re-introduced by the +reversion of W. + +However, you can avoid these problems if you recreate the entire branch, +including commit A: + + A'---B'---C' <-- completely rewritten branch + / + P---o---o---M---x---x---W---x + \ / + A---B---C + +You can merge A'-B'-C' into the mainline branch without worrying about first +reverting W. Mainline's history would look like this: + + A'---B'---C'------------------ + / \ + P---o---o---M---x---x---W---x---M2 + \ / + A---B---C + +But if you don't actually need to change commit A, then you need some way to +recreate it as a new commit with the same changes in it. The rebase command's +--no-ff option provides a way to do this: + + $ git rebase [-i] --no-ff P + +The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the +SHA IDs will be different) even if in the interactive case you only actually +modify commit B. You can then merge this new branch directly into the mainline +branch and be sure you'll get all of the branch's changes. + +You can also use --no-ff in cases where you just add extra commits to the topic +to fix it up. Let's revisit the situation discussed at the start of this howto: + + P---o---o---M---x---x---W---x + \ / + A---B---C----------------D---E <-- fixed-up topic branch + +At this point, you can use --no-ff to recreate the topic branch: + + $ git checkout E + $ git rebase --no-ff P + +yielding + + A'---B'---C'------------D'---E' <-- recreated topic branch + / + P---o---o---M---x---x---W---x + \ / + A---B---C----------------D---E + +You can merge the recreated branch into the mainline without reverting commit W, +and mainline's history will look like this: + + A'---B'---C'------------D'---E' + / \ + P---o---o---M---x---x---W---x---M2 + \ / + A---B---C |